(1) I'm obligated to pay you back $10 tonight.
(2) I can't pay you back $10 tonight (e.g., I just gambled away
my last dime).

Since this puzzle typically involves some notion of possibility, let
us represent the above sentences in KTd, which includes SDL,
but also has a possibility operator:

(1′) OBp
(2′) ~◊p

(1) and (2) appear to be consistent. It seems to be a sad fact that
often, people are unable to fulfill their financial
obligations, just as it seems to be a truism that financial
obligations are obligations. But in KTd, it is a theorem
that OBp → ◊p. So we derive a
contradiction from this symbolization and the assumption that
(1′) and (2′) are true.

A variant example is:

(1) I owe you ten dollars, but I can't pay you back.
(2) I'm obligated to pay you ten dollars, but I can't.

(2) seems to follow from (1), and (1) hardly seems contradictory, since
owing money clearly does not entail being able to pay the money owed.
Thomason 1981b suggests a distinction between deliberative contexts of
evaluation and judgmental contexts, where in the latter context
evaluations such as (1) above need not satisfy Kant's law since,
roughly, we go back in time and evaluate the present in terms of where
things would now be relative to optimal past options that were
accessible but no longer are.